This invention relates generally to electronic devices that use thermionic emission of electrons and, more particularly, to heater assemblies for heating cathodes to provide the thermionically emitted electrons.
As it is known in the art, vacuum devices such as travelling wave tubes and other microwave devices generally include a cathode which is heated to produce thermionically emitted electrons. Generally, the cathode is indirectly heated by use of a heater assembly which houses a filament. A current is supplied to the filament to raise the temperature of the filament to a temperature in the range of at least 900.degree. C. to 1200.degree. C. The filament of the heater assembly provides thermal energy required to raise the temperature of the cathode electrode to produce sufficient electron emission from the cathode to power the tube.
The heater assembly generally includes a filament wire which is coiled about a region and is maintained in a position relative to the cathode throughout the operating life of the microwave tube. One common approach to providing such heater assemblies is to provide a coiled filament wire supported by a dielectric potting. Generally, the dielectric used for the potting must be a relatively refractory material such as a ceramic in order to withstand the relatively high temperatures typically provided by the filament. Since the thermal transfer properties between the heater filament and the cathode are a critical characteristic to determine overall thermionic emission of electrons, the physical arrangement the heater and the cathode must remain substantially constant over the operating life of the tube. Any variation in the position of the heater filament with respect to the cathode will cause a concomitant change in the temperature of the emitting surface of the cathode and thus a change in the rate of electron emission from the surface of the cathode. Electron emission from such a surface is very sensitive to temperature variations.
Further, the cathode heater assemblies are subject to rapid changes in temperature which can cause failure of the assemblies by cracking of the potting material. Moreover, in many applications of these tubes, such as in airborne applications, the tubes are subjected to high levels mechanical vibration and mechanical shock which likewise can have adverse effects on the potting materials and can cause failure of the heater.
Generally, techniques used to provide suitable pottings for these tubes rely upon encapsulation of the coiled filament which often provide pottings having less than 80% of theoretical density of the potting material and, furthermore, often provide pottings having voids or spaces in the potting which can act as fracture centers during subsequent operation of the tube. The potting approach is also relatively expensive since a significant amount of manual labor, as well as reworking of the potting assembly is often required to provide a useable filament heater.